Solar cells are providing widespread benefits to society by converting essentially unlimited amounts of solar energy into useable electrical power. As their use increases, certain economic factors become important, such as high-volume manufacturing and efficiency.
Solar radiation is assumed to preferentially illuminate one surface of a solar cell, usually referred to as the front side. In order to achieve a high energy conversion efficiency of incident photons into electric energy, an efficient absorption of photons within a silicon substrate is important. This can be achieved by a good surface texturing and antireflection coating on the front side and a low parasitic absorption within all layers except the substrate itself. Further, it can be important to provide a reflecting layer at the back of the cell to improve internal light trapping. Another important parameter for high solar cell efficiency is the shading of the front surface by metal electrodes. In general, an optimized metal grid requires a tradeoff of losses between shading and electrical resistance of the metal structure. The optimization for efficiency of the solar cell requires a grid with very narrow fingers and short distances between those fingers, which should have a high electrical conductivity. A practical method to form this structure is the subject of this invention.
Solar cell production may use, e.g., screen printing technology to print the electrode on the front surface. A silver paste can be printed over a silicon nitride antireflection coating and fired through the coating in a high temperature process. This is a short process, however, certain inherent properties of this approach include a comparatively broad line width in excess of 50 um (typically about 100 um) and a fairly low line conductivity of the metal grid due to the use of several non-metallic components in the printed paste. Moreover, the firing process results in a penetration of the metal paste ingredients through the antireflection layer into the substrate where increased recombination occurs. This holds for both cases of a front junction device where the pn-junction can be severely damaged by unwanted penetration of the space charge region as well for back junction devices where the front surface recombination is increased and significantly reduces the collection efficiency of the back junction emitter.